Detecting duplicate and near-duplicate files

ABSTRACT

Improved duplicate and near-duplicate detection techniques may assign a number of fingerprints to a given document by (i) extracting parts from the document, (ii) assigning the extracted parts to one or more of a predetermined number of lists, and (iii) generating a fingerprint from each of the populated lists. Two documents may be considered to be near-duplicates if any one of their fingerprints match.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/608,468, entitled “DETECTING DUPLICATE AND NEAR-DUPLICATE FILED”,filed on Jun. 27, 2003, and listing William Pugh and Monika H. Henzingeras inventors, which is a continuation of U.S. patent application Ser.No. 09/768,947, entitled “DETECTING DUPLICATE AND NEAR-DUPLICATE FILED”,filed on Jan. 24, 2001 and listing William Pugh and Monika H. Henzingeras inventors, which issued as U.S. Pat. No. 6,658,423. Benefit to theseapplications is claimed under 35 U.S.C. § 120. These applications areexpressly incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention concerns information management and retrieval ingeneral. More specifically, the present invention concerns detecting,and optionally removing, duplicate and near-duplicate information orcontent, such as in a repository of documents to be searched forexample.

2. Related Art

In the following, the term “document(s)” should be broadly interpretedand may include content such as Web pages, text files, multimedia files,object features, link structure, etc. Also, it should be noted that whennear-duplicate documents are detected, exact duplicate documents willalso be detected as a consequence (though such exact duplicates mightnot necessarily be distinguished from near-duplicates).

Detecting near-duplicate documents has many potential applications. Forexample, duplicate or near-duplicate documents may indicate plagiarismor copyright infringement. One important application of near-duplicatedocument detection is in the context of information storage andretrieval.

Efficient techniques to detect documents that are exact duplicatesexist. Detecting whether or not documents are near-duplicates is moredifficult, particularly in large collections of documents. For example,the Internet, collectively, includes literally billions of “Web site”documents.

Sources of duplicate and near-duplicate documents on the Internet areintroduced in § 1.2.1 below. Then, problems that these duplicate andnear-duplicate documents raise, both for end-users and for entitiesassisting end-users, are introduced in § 1.2.2 below. Finally, previoustechniques for detecting duplicate and near-duplicate documents in thecontext of large document collections, as well as perceived shortcomingsof such techniques, are introduced in § 1.2.3 below.

§ 1.2.1 Sources of Duplicate and Near-Duplicate Documents on theInternet

On the Internet, the World Wide Web (referred to as “the Web”) mayinclude the same document duplicated in different forms or at differentplaces. (Naturally, other networks, or even stand alone systems, mayhave duplicate documents.) Sources of such duplication are introducedhere.

First, some documents are “mirrored” at different sites on the Web. Suchmirroring is used to alleviate potential delays when many users attemptto request the same document at the same time, and/or to minimizenetwork latency (e.g., by caching Web pages locally).

Second, some documents will have different versions with differentformatting. For example, a given document may have plain text and HTML(hyper-text markup language) versions so that users can render ordownload the content in a form that they prefer. As more and moredifferent devices (e.g., computers, mobile phones, personal digitalassistants, etc.) are used to access the Internet, a given document mayhave more and more different versions with different formatting (textonly, text plus other media, etc.).

Third, documents are often prepended or appended with informationrelated to its location on the Web, the date, the date it was lastmodified, a version, a title, a hierarchical classification path (e.g.,a Web page may be classified under more than one class within thehierarchy of a Web site), etc. An example of such near-duplicatedocuments is illustrated in § 4.4 below, with reference to FIGS. 13through 18.

Fourth, in some instances a new document is generated from an existingdocument using a consistent word replacement. For example, a Web sitemay be “re-branded” for different audiences by using word replacement.

Finally, some Web pages aggregate or incorporate content available fromanother source on the Web.

§ 1.2.2 Problems Raised by Duplicate and Near-Duplicate Documents

Duplicate and near-duplicate documents raise potential problems for bothpeople accessing information (e.g., from the Web) and entities helpingpeople to access desired information (e.g., search engine companies).These potential problems are introduced below.

Although people continue to use computers to enter, manipulate and storeinformation, in view of developments in data storage, internetworking(e.g., the Internet), and interlinking and cross referencing ofinformation (e.g., using hyper-text links), people are using computers(or more generally, information access machines) to access informationto an ever increasing extent.

Search engines have been employed to help users find desiredinformation. Search engines typically search databased content or “Websites” pursuant to a user query. In response to a user's query, arank-ordered list, which typically includes brief descriptions of theuncovered content, as well as hyper-texts links (i.e., text, havingassociated URLs) to the uncovered content, is returned. Therank-ordering of the list is typically based on a match between wordsappearing in the query and words appearing in the content.

From the perspective of users, duplicate and near-duplicate documentsraise problems. More specifically, when users submit a query to a searchengine, most do not want links to (and descriptions of) Web pages whichhave largely redundant information. For example, search enginestypically respond to search queries by providing groups of ten results.If pages with duplicate content were returned, many of the results inone group may include the same content. Thus, there is a need for atechnique to avoid providing search results associated with (e.g.,having links to) Web pages having duplicate content.

From the perspective of entities hosting search engines, duplicate andnear-duplicate documents also raise problems—giving end-users what theywant, being one of them. To appreciate some of the other potentialproblems raised by duplicate and near-duplicate documents, some searchengine technology is introduced first.

Most search engines perform three main functions: (i) crawling the Web;(ii) indexing the content of the Web; and (iii) responding to a searchquery using the index to generate search results. Given the large amountof information available, these three main functions are automated to alarge extent. While the crawl operation will associate words or phraseswith a document (e.g., a Web page), the indexing operation willassociate document(s) (e.g., Web page(s)) with words or phrases. Thesearch operation then (i) uses that index to find documents (e.g., Webpages) containing various words of a search query, and (ii) ranks ororders the documents found in accordance with some heuristic(s).

Recall that the Web may include the same documents duplicated indifferent forms or at different places on the Web. For example, asintroduced in § 1.2.1 above, documents may be “mirrored” at differentsites on the Web, documents may have a number of different formats sothat users can render or download the content in a form that theyprefer, documents may have a different versions with differentinformation prepended or appended, some documents may have beengenerated from others using consistent word replacement, and somedocuments may aggregate or incorporate documents available from anothersource on the Web. It would be desirable to eliminate such duplicates ornear-duplicates. Aside from eliminating duplicate or near-duplicatedocuments to meet user expectations and wishes, eliminating duplicate ornear-duplicate documents is desirable to search engine hosting entitiesto (i) reduce storage requirements (e.g., for the index and datastructures derived from the index), and (ii) reduce resources needed toprocess indexes, queries, etc.

In view of the foregoing, techniques to detect (and eliminate)near-duplicate documents are needed.

§ 1.2.3 Known Techniques for Detecting Duplicate and Near-DuplicateDocuments

Some previous techniques for detecting duplicate and near-duplicatedocuments involve generating so-called “fingerprints” for elements(e.g., paragraphs, sentences, words, or shingles (i.e., overlappingstretches of consecutive words)) of documents. See, e.g., the articles:A. Z. Broder, “On the Resemblance and Containment of Documents,”Proceedings of Compression and Complexity of Sequences 1997, pp. 21-27,IEEE Computer Society (1988); and S. Brin et al., “Copy DetectionMechanisms for Digital Documents,” Proceedings of the ACM SIGMOD AnnualConference, San Jose 1995 (May 1995). Some or all of the generatedfingerprints could be used in a duplicate/near-duplicate determination.More specifically, two documents would be considered to benear-duplicates if they share more than a predetermined number (at leasttwo, and generally much higher) of fingerprints. That is, such methodsdetermine when documents share multiple common fingerprints. Generally,if the predetermined number is too low, too many false positives wouldbe generated.

For a large collection of documents (e.g., billions of documents to beindexed by a search engine), this determination becomes quite expensive,computationally and in terms of storage. See, e.g., the article, M. Fanget al., “Computing Iceberg Queries Efficiently,” Proc. 24^(th) Int'l.Conf. On Very Large Databases, pp. 299-310 (1998). This problem is noteasily overcome. For example, it is not especially useful to“preprocess” the representations of such documents used in the Brodertechnique to eliminate from further consideration, fingerprints known tobe unique. This is because even documents with non-unique fingerprints(i.e., documents remaining after such preprocessing) may, nonetheless,have no near-duplicate documents. Thus, a betterduplicate/near-duplicate determination technique is needed.

SUMMARY OF THE INVENTION

At least some embodiments consistent with the present invention maydetermine whether two documents are near-duplicates by first generatingat least two different fingerprints for each of the two documents.Whether or not the two documents are near-duplicate documents may thenbe determined by (1) determining whether or not any one of the at leasttwo fingerprints of a first of the two documents matches any one of theat least two fingerprints of a second of the two documents, and (2) ifit is determined that any one fingerprint of the at least twofingerprints of the first of the two documents does match any onefingerprint of the at least two fingerprints of the second of the twodocuments, then concluding that the two documents are near-duplicates.

In at least some embodiments consistent with the present invention, thedetermination of whether or not the two documents are near-duplicates isthen used in (A) an act of serving search results corresponding todocuments, (B) an act of crawling documents, (C) an act of indexingdocuments, and/or (D) an act of fixing a broken link to at least one ofthe two documents.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a high-level block diagram of an environment in which at leastsome aspects of the present invention may be used.

FIG. 2 is a process bubble diagram of an advanced search facility inwhich at least some aspects of the present invention may be used.

FIG. 3 is a process bubble diagram that illustrates some operations thatmay be performed by the present invention.

FIG. 4 is a high-level flow diagram of an exemplary method that may beused to effect an extraction operation.

FIG. 5 is a high-level flow diagram of an exemplary method that may beused to effect a list population operation.

FIG. 6 is a high-level flow diagram of an exemplary method that may beused to effect a fingerprint generation operation.

FIG. 7 is a high-level flow diagram of an exemplary method that may beused to effect a near-duplicate detection operation.

FIG. 8 is a high-level flow diagram of an exemplary method that may beused to effect a cluster determination operation.

FIG. 9 is a high-level flow diagram of an exemplary method that may beused to effect a query-responsive near-duplicate detection operation.

FIG. 10 is a high-level block diagram of a machine that may be used toeffect various operations of the present invention.

FIG. 11 is an example illustrating an operation of an exemplaryextraction operation.

FIGS. 12A and 12B, collectively, provide an example illustrating anoperation of an exemplary list population operation.

FIG. 13 illustrates a Web page of results to a search query.

FIGS. 14 through 18 illustrate near-duplicate documents that would be(related to snippets and hyper-text links) returned if near-duplicatedocuments were not detected and eliminated.

DETAILED DESCRIPTION

The present invention involves novel methods, apparatus and datastructures for identifying near-duplicate documents. The followingdescription is presented to enable one skilled in the art to make anduse the invention, and is provided in the context of particularapplications and their requirements. Various modifications to thedisclosed embodiments will be apparent to those skilled in the art, andthe general principles set forth below may be applied to otherembodiments and applications. Thus, the present invention is notintended to be limited to the embodiments shown and the inventor regardshis invention as the following disclosed methods, apparatus, articles ofmanufacturers, and data structures and any other patentable subjectmatter to the extent that they are patentable.

In the following, environments in which the present invention may beemployed are introduced in § 4.1. Then, functions that may be performedby the present invention are introduced in § 4.2. Then, operations, datastructures, methods and apparatus that may be used to effect thosefunctions are described in § 4.3. Thereafter, examples of how exemplaryparts of the present invention may operate is described in § 4.4.Finally, some conclusions about the present invention are set forth in §4.5.

§ 4.1 Exemplary Environments in which Invention may Operate

The following exemplary embodiments are presented to illustrate examplesof utility of the present invention and to illustrate examples ofcontexts in which the present invention may operate. However, thepresent invention can be used in other environments and its use is notintended to be limited to the exemplary environment 100 and searchfacility 200 introduced below with reference to FIGS. 1 and 2,respectively.

FIG. 1 is a high-level block diagram of an environment 100 in which atleast some aspects of the present invention may be used. Thisenvironment 100 may be a network (such as the Internet for example) 160in which an information access facility (client) 110 is used to renderinformation accessed from one or more content providers (servers) 180. Asearch facility (server) 130 may be used by the information accessfacility 110 to search for content of interest.

The information access facility 110 may include a browsing operation 112which may include a navigation operation 114 and a user interfaceoperation 116. The browsing operation 112 may access the network 160 viainput/output interface operations 118. For example, in the context of apersonal computer, the browsing operation 112 may be a browser (such as“Internet Explorer” from Microsoft Corporation of Redmond, Wash., or“Netscape” from Netscape Communications, Inc.) and the input/outputinterface operations may include a modem or network interface card (orNIC) and networking software. Other examples of possible informationaccess facilities 110 include untethered devices, such as personaldigital assistants and mobile telephones for example, set-top boxes,kiosks, etc.

Each of the content providers 180 may include stored resources (alsoreferred to as content) 136, a resource retrieval operation 184 thataccesses and provides content in response to a request, and input/outputinterface operation(s) 182. These operations of the content providers180 may be effected by computers, such as personal computers or serversfor example. Accordingly, the stored resources 186 may be embodied asdata stored on some type of storage medium such as a magnetic disk(s),an optical disk(s), etc. In this particular environment 100, the term“document” may be interpreted to include addressable content, such as aWeb page for example.

The search facility 130 may perform crawling, indexing/sorting, andquery processing functions. These functions may be performed by the sameentity or separate entities. Further, these functions may be performedat the same location or at different locations. In any event, at acrawling facility 150, a crawling operation 152 gets content fromvarious sources accessible via the network 160, and stores such content,or a form of such content, as indicated by 154. Then, at an automatedindexing/sorting facility 140, an automated indexing/sorting operation142 may access the stored content 154 and may generate a content index(e.g., an inverted index, to be described below) and content ratings(e.g., PageRanks, to be described below) 140. Finally, a queryprocessing operation 134 accepts queries and returns query results basedon the content index (and the content ratings) 140. The crawling,indexing/sorting and query processing functions may be performed by oneor more computers.

Although the present invention may be used with a number of differenttypes of search engines, the present inventor anticipates that it willbe used with an advanced search facility, such as the one presentlyavailable from Google, Inc. of Mountain View, Calif. FIG. 2 is a processbubble diagram of such an advanced search facility 200 in which at leastsome aspects of the present invention may be used.

The advanced search facility 200 illustrated in FIG. 2 performs threemain functions: (i) crawling; (ii) indexing/sorting; and (iii)searching. The horizontal dashed lines divide FIG. 2 into three partscorresponding to these three main functions. More specifically, thefirst part 150′ corresponds to the crawling function, the second part140′ corresponds to the indexing/sorting function, and the third part134′ corresponds to the search (or query processing) function. (Notethat an apostrophe “′” following a reference number is used to indicatethat the referenced item is merely one example of the item referenced bythe number without an apostrophe.) Each of these parts is introduced inmore detail below. Before doing so, however, a few distinguishingfeatures of this advanced search facility 200 are introduced.

The advanced search facility uses the link structure of the Web, as wellas other techniques, to improve search results. (See, e.g., the articleS. Brin and L. Page, “The Anatomy of a Large-Scale Hypertextual SearchEngine,” Seventh International World Wide Web Conference, Brisbane,Australia. Incorporated herein by reference.)

Referring back to FIG. 2, the three main parts of the advanced searchengine 200 are now described further.

The crawling part 150′ may be distributed across a number of machines. Asingle URLserver (not shown) serves lists of uniform resource locations(“URLs”) 206 to a number of crawlers. Based on this list of URLs 206,the crawling operation 202 crawls the network 160′ and gets Web pages208. A pre-indexing operation 210 may then generate page rankings 212,as well as a repository 214 from these Web pages 208. The page rankings212 may include a number of URL fingerprint (i.e., a unique value),PageRank value (as introduced above) pairs. The repository 214 mayinclude URL, content type and compressed page triples.

Regarding the indexing/sorting part 140′, the indexing/sortingoperations 220 may generate an inverted index 226. The indexing/sortingoperations 220 may also generate page ranks 228 from the citationrankings 212. The page ranks 228 may include document ID, PageRank valuepairs.

Regarding the query processing part 134′, the searching operations 230may be run by a Web server and may use a lexicon 232, together with theinverted index 226 and the PageRanks 228, to generate query results inresponse to a query. The query results may be based on a combination of(i) information derived from PageRanks 228 and (ii) information derivedfrom how closely a particular document matches the terms contained inthe query (also referred to as the information retrieval (or “IR”)component). Having described exemplary environments in which the presentinvention may be used, functions that may be performed by the presentinvention are now introduced in § 4.2 below.

§ 4.2 Functions that may be Performed by the Present Invention

One goal of the present invention is to provide a better near-duplicatedetermination technique. Such a technique should be less computationallyexpensive than the prior techniques introduced in § 1.2.3 above. Such atechnique should not generate too many false positives (i.e., falseindications that documents are duplicates or near-duplicates when, infact, they are not). The present invention should also be able to detectdocuments that are duplicates or near-duplicates, but that include aconsistent word replacement, as introduced in § 1.2.1 above.

At a high level, the present invention may function to detectnear-duplicate documents (e.g., Web pages). To reiterate, it will bepresumed that detecting near-duplicate document will necessarily alsodetect exact duplicate documents. Therefore, when the term“near-duplicate detection” is used, it will be understood that exactduplicates will also be detected, though not necessarily identified as“exact”, as opposed to near, duplicates. The present invention maydetect near-duplicate documents by (i) for each document, generatingfingerprints, (ii) preprocessing (optionally) the fingerprints toeliminate those that only occur in one document, and (iii) determiningnear-duplicate documents based on the (remaining) fingerprints. The actof generating fingerprints for each document may be effected by (i)extracting parts (e.g., words) from the documents, (ii) hashing each ofthe extracted parts to determine which of a predetermined number oflists is to be populated with a given part, and (iii) for each of thelists, generating a fingerprint.

In response to the detected duplicate documents, the present inventionmay also function to eliminate duplicate documents (e.g., keeping theone with best PageRank, with best trust of host, that is the mostrecent) Alternatively, the present invention may function to generateclusters of near-duplicate documents, in which a transitive property isassumed (i.e., if document A is a near-duplicate of document B, anddocument B is a near-duplicate of document C, then document A isconsidered a near-duplicate of document C). Each document may have anidentifier for identifying a cluster with which it is associated. Inthis alternative, in response to a search query, if two candidate resultdocuments belong to the same cluster and if the two candidate resultdocuments match the query equally well (e.g., have the same title and/orsnippet) if both appear in the same group of results (e.g., first page),only the one deemed more likely to be relevant (e.g., by virtue of ahigh PageRank, being more recent, etc.) is returned.

In the context of a search engine, the present invention may also beused during a crawling operation 202 to speed up the crawling and tosave bandwidth by not crawling near-duplicate Web pages or sites, asdetermined from documents uncovered in a previous crawl. Further, byreducing the number of Web pages or sites crawled, the present inventioncan be used to reduce storage requirements for other “downstream” storeddata structures. Alternatively, the present invention may be used aftercrawling such that, if there are more two or more near duplicatedocuments, only one is indexed. The present invention can instead beused later, in response to a query, in which case a user is not annoyedwith near-duplicate search results. The present invention may also beused to “fix” broken links. That is, if a document (e.g., a Web page)doesn't exist (at a particular location or URL) anymore, a link to anear-duplicate page can be provided.

Having introduced functions that may be performed by the presentinvention, exemplary operations, data structures, methods and apparatusfor effecting these functions are described in § 4.3 below.

§ 4.3 Exemplary Operations, Data Structures, Methods and Apparatus forEffecting Functions that may be Performed by the Present Invention

In the following, exemplary operations that may be performed by thepresent invention, and exemplary data structures that may be used by thepresent invention, are introduced in § 4.3.1 with reference to FIG. 3.Then, exemplary methods for effecting such operations are described in §4.3.2 with reference to FIGS. 4 through 9. Finally, exemplary apparatusthat may be used to effect the exemplary processes and store theexemplary data structures are described in § 4.3.3 with reference toFIG. 10.

§ 4.3.1 Exemplary Operations and Data Structures

FIG. 3 is a process bubble diagram that illustrates operations that maybe performed by the present invention. A near-duplicate detectionfingerprint generation operation 330 may be used to generate a pluralityof fingerprints 365 for each of a number of documents 320/214′. Morespecifically, an extraction operation 340 may be used to extract parts(e.g., words, terms, numbers, etc.) from documents, and associate adocument identifier with the extracted parts, as indicated by 345 (eachrecord including a document identifier and associated extracted partsmay be referred to as “document extracted parts”). Exemplary methods foreffecting the extraction operation 340 are described in § 4.3.2.1 belowwith reference to FIG. 4. An example illustrating the operation of anexemplary extraction operation 340 is provided in § 4.4 below withreference to FIG. 11. The documents 320 may be accepted from any source,such as the repository 214′ of the search engine 200 of FIG. 2. Thedocuments 320 to be processed may be identified in a list 310.

A list population operation 350 may be used to process the documentextracted parts 345 and place each extracted part 345 into one of apredetermined number (e.g., four) of lists 355 (each record including adocument identifier and associated lists may be referred to as “documentlists”). In the context of comparing the text of web pages, it isexpected that three to eight lists, will yield good results. Goodresults have been obtained in this context using three lists and fourlists. In the context of a context vectors of words for use in a“thesaurus” application, good results have been obtained with threelists and with nine lists. Thus, a document identifier will beassociated with a predetermined number of lists, each of the lists beingpopulated (though some lists may be empty) with extracted documentparts. The list population operation 350 may use a hash function that isrepeatable, deterministic, and not sensitive to state. For example, theword “the” will always be sent to the same list, without regard to thenumber of times it occurs in a document, and without regard to whichdocument it occurs in. Exemplary methods for effecting the listpopulation operation 350 are described in § 4.3.2.2 below with referenceto FIG. 5. An example illustrating the operation of an exemplary listpopulation operation 350 is provided in § 4.4 below with reference toFIGS. 12A and 12B.

A fingerprint generation operation 360 may be used, for each document,to process the populated document lists 355 to generate a fingerprintfor each of the lists (each record including a document identifier andassociated fingerprints may be referred to as “document fingerprints”).Exemplary methods for effecting the fingerprint generation operation 360are described in § 4.3.2.3 below with reference to FIG. 6. Thus, adocument identifier will be associated with a predetermined number(e.g., four) of fingerprints as indicated by 365. The predeterminednumber of fingerprints may correspond to the predetermined number oflists. The fingerprint generation operation 360 should be designed suchthat it is very unlikely that two different lists would produce the samefingerprint, but such that two identical lists will always generate thesame fingerprint.

Once the document fingerprints 365 for each of a number of documents isdetermined, a near-duplicate detection operation 376 may be used todetermine whether or not any two documents are “near-duplicates”. In oneembodiment of the invention, if two documents have any one fingerprintin common, they are considered to be “near-duplicates”. If each of thefingerprints are the same, the two documents could be considered to be“exact-duplicates”. Exemplary methods for effecting the near-duplicatedetection operation 376 are described in § 4.3.2.4 below with referenceto FIG. 7.

Having described operations that may be performed in an exemplaryembodiment of the present invention, some optimizations to suchoperations and alternative operations are now introduced.

§ 4.3.1.1 Reducing the Size of the Collection of Document Fingerprints

A very large collection of documents 320 will generate a very largecollection of document fingerprints 365. Reducing the size of thecollection of document fingerprints 365 without impacting the efficacyof the near-duplicate detection operation 376 is clearly desirable. Oneor more fingerprint preprocessing operations 372 may be used for thispurpose. For example, a fingerprint preprocessing operation 372 may beused to eliminate fingerprints that only occur in a single document,leaving only those fingerprints occurring in more than one document 374.Such fingerprints may be removed since, in accordance with oneembodiment of the near-duplicate detection operation 376, they willnever indicate a near-duplicate document because they have no matchingfingerprint in another document. Note that if such a fingerprintpreprocessing operation 372 is used, then the near-duplicate detectionoperation 376 may use a reduced collection of document fingerprints 374,rather than the full collection 365. Alternatively, a near-duplicatedetection operation other than that of the present invention may beused. That is, the present invention may be used solely for generating areduced collection of document fingerprints 374. Any near-duplicatedetection technique may then operate on such a reduced collection.

Similarly, the techniques of the present invention can be used as apre-filtering step in a more careful and (e.g., computationally and/orstorage) expensive near-duplicate determination technique. In this way,pairs of objects (e.g., documents) indicated by the technique of thepresent invention as being near-duplicates would be checked using thesecond, more careful, near-duplicate determination technique. Pairsindicated by the technique of the present invention as not beingnear-duplicates would simply be discarded. Naturally, in such apre-filtering application, the present invention could be tuned to erron the side of generating false positive near-duplicate indications.

§ 4.3.1.2 Generating and Using Clusters of Near-Duplicate Documents

The notion of near-duplicate documents can be extended to have atransitive property. That is, in such a case, if document A is anear-duplicate of document B, which is a near-duplicate of document C,then A is considered to be a near-duplicate of document C (even if anear-duplicated detection operation 376 would not otherwise considerdocuments A and C to be near-duplicates). A document clusterdetermination operation 382 (e.g., a known union-find technique) may beused to define clusters of documents in this way. Note that a documentwill only belong to a single cluster—if it belonged to two separateclusters, the two clusters could be combined into one by virtue of thecommon document. A data structure 383 associating a document (e.g., adocument identifier) with a cluster (e.g., a cluster identifier) may beused to reflect the clusters determined. These clusters ofnear-duplicate documents may be used as follows. Exemplary methods thatmay be used to effect this clustering operation 382 are described in §4.3.2.5 below with reference to FIG. 8.

In the context of a search application, a search operation will oftenreturn search results in response to a query. (See, e.g., FIG. 2.) Thesearch results may be grouped into a predetermined number (e.g., ten) ofsearch results (e.g., snippets of documents with hyper-text links to thedocuments). A query-responsive duplicate detection operation 384 mayfunction as follows. If the search results include two documents thatbelong to the same cluster, and those two documents match (in terms oftraditional information retrieval) the query equally well, then only thehigher quality document (e.g., more recent, higher PageRank, morerelevant, etc.) is kept, the other being eliminated. Exemplary methodsthat may be used to effect the query-responsive duplicate detectionoperation 384 are described in § 4.3.2.5 below with reference to FIG. 9.An example illustrating the operation of an exemplary query-responsiveduplicate detection operation 384 is described in § 4.4 below withreference to FIGS. 13 through 18.

§ 4.3.2 Exemplary Methods

Exemplary methods that may be used to effect some of the operationsintroduced in § 4.3.2 above, are now described in §§ 4.3.2.1 through4.3.2.5.

§ 4.3.2.1 Exemplary Extraction Methods

FIG. 4 is a high-level flow diagram of an exemplary method 340′ that maybe used to effect the extraction operation 340. As indicated by block410 unit type (e.g., word, sentence, character, paragraph, section,etc.), part size (as a number of units) and part overlap (e.g., nooverlap or shingles defined by an overlap having a predetermined ofunits) may be accepted. These parameters are tunable, but once set,should be applied consistently across all documents to be checked todetermine whether or not any are near-duplicates. As indicated in block420, a document is accepted. Then, as indicated by the loop 430-450through all parts of the document, parts are extracted and stored inassociation with a unique document identifier (referred to as “doc ID”).After all of the parts of a document have been processed, the method340′ is left via RETURN node 460. Note that the extraction method 340′may be applied to some or all documents of a collection to be analyzed.

Referring back to block 440, extraction may be effected using any one ofa number of known techniques. Referring back to block 410, the parts tobe extracted from a document may be sections, paragraphs, sentences,words, or characters for example. See, e.g., the article, S. Brin etal., “Copy Detection Mechanisms for Digital Documents,” Proceedings ofthe ACM SIGMOD Annual Conference, San Jose (May 1995), downloaded fromwww-db.Stanford.edu/˜sergey/copy.html on Nov. 27, 2000 (hereafterreferred to as “the Brin article”).

Before extraction occurs, a document may be preprocessed to removeformatting information and non-textual components thereby generating aso-called “canonical form” document consisting of ASCII characters withwhite-space separating words, punctuation separating sentences, etc.See, e.g., the Brin article.

If the document is not a text document, or if it includes non-textualcomponents, other features may be extracted using known techniques.Further, the techniques of the present invention can be used in otherapplications in which some measure of similarity is needed. For example,in the context of providing a thesaurus operation, words which havesimilar (e.g., near-duplicate) “context vectors” may be consideredsynonyms. A context vector of a given word may be thought of as wordsthat frequently (e.g., at least a predetermined number of times) appearclose to (e.g., within a predetermined proximity of) the word in thegiven collection of documents.

The extraction operation 340 may be modified so that short or commonwords or terms (e.g., stop words) are not processed (i.e., ignored orremoved by preprocessing).

The extraction operation 340 may be modified so that short documents(e.g., documents with 50 words or less) are not processed at all. Forexample, standard error pages (e.g., informing a user about a dead link,etc.) are typically short, and should not be processed.

An exemplary operation of an exemplary extraction operation 340 isdescribed in § 4.4 below with reference to FIG. 11.

§ 4.3.2.2 Exemplary List Population Methods

FIG. 5 is a high-level flow diagram of an exemplary method 350′ that maybe used to effect the list population operation 350. As indicated byblock 510, a predetermined number of lists to be populated is accepted.This list number parameter (e.g., four) is tunable, but once set, shouldbe applied consistently across all documents to be checked to determinewhether or not any are near-duplicates. Further, as indicated by block520, document parts extracted from a document are accepted. Then, asindicated by the loop 530-550 through all of the accepted parts, acurrent part is hashed to determine which of the predetermined number oflists to populate with the current part, and to populate the determinedlist accordingly.

As is well known in the art, hashing functions are used to compress data(e.g., a variable-size input) in a deterministic way, to generate a hashvalue (e.g., a fixed-size string). That is, given data always hashes tothe same hash result.

Hash functions may be one-way (non-reversible). That is, given a hashvalue, it is impractical to find the data from which the hash value wasgenerated. For purposes of the present invention, the hash function neednot be non-reversible.

Hash functions may be strongly collision-free. That is, for a stronglycollision-free hash function H, it is infeasible to find any twodifferent inputs x and y, such that H(x)=H(y). However, for purposes ofthe present invention, since the number of lists which the hash functionis used to populate is rather limited (e.g., four) and is, in any event,much less than the number of possible different document parts (e.g.,words or sentences) to be hashed, the hash function used by the listpopulation operation 350 need not be strongly collision-free. Note,however, that ranges of hash values or different hash values can bemapped to a single list.

Once all of the parts are processed by the loop 530-550, as indicated byblock 560, the document identifier may be associated with the populatedlists before the method 350 is left via RETURN node 570.

Note that as the number of lists increases, the chances of two documentshaving identical lists increases and consequently, a near-duplicatedetermination increases. Given this fact, the number of lists to bepopulated with document elements may be changed and adjusted, asfollows, so that two different documents do not share any commonfingerprints. If the number “x” of lists increases, the expected numberof document differences (or changes from one document to obtain thesecond document) needed before two documents no longer share any commonfingerprints increases.

An exemplary operation of an exemplary list population operation 350 isdescribed in § 4.4 below with reference to FIGS. 12A and 12B.

In a more fundamental alternative list population operation, rather thanhaving each part (e.g., word) go into exactly one list, each part (e.g.,word) can go into zero, one, or more lists. For each list L_(i), aseparate hash function H_(i) would be provided for determining whetheror not a part (e.g., word) should go into the i^(th) list (L_(i)). Thatis, if H_(i)(part_(n))=true, then list L_(i) would include the n^(th)part (part_(n)). The hash functions for each list should be independent.

In the foregoing alternative list population operation, the hashfunction and/or the number of lists can be tuned in accordance with thefollowing principles. Assume that the probability that a particularhashing function returns a “true” value in response to a part (e.g.,word) is “p”. Accordingly, the probability that a particular list willchange given the part (e.g., a word) is p, and the probability that thegiven list will not change given the part (e.g., word) is 1−p. Theprobability that the given list will not change given a number “k” ofparts (e.g., words) is therefore (1−p)^(k). Conversely, the probabilitythat the given list will change given a number “k” of parts (e.g.,words) is 1−(1−p)^(k). If there are x lists, the chance that twodocuments having k different parts (e.g., words) will have a common list(and therefore share a fingerprint in common, and therefore beconsidered to be near-duplicates) is 1−(1−(1−p)^(k))^(x). The hashingfunctions (and thus, p) and the number “x” of lists can be adjustedbased on the foregoing relationship.

Further note that in the foregoing alternative list populationoperation, as the number of parts increases (e.g., if the part is aword, then as the number of words in a document increases), the chancesof a change to any of the lists increases (assuming a fixed number “x”of lists). Consequently, the chances of determining that two documentsare near-duplicates will decrease (assuming a fixed number “x” oflists). To compensate for this sensitivity to document size (or moregenerally, to the number of parts extracted), the probability(ies) “p”associated with the hash function(s) may be chosen to be determinedbased on the size of a document. More specifically, by slowly decreasing“p” for larger documents (thereby changing the hashing function H_(i) aswell, so that for larger documents, the set of parts (e.g., words) forwhich H_(i) returns “true” is a subset of the set it would return forsmaller documents), the ability to find near-duplicate documents couldbe preserved (or would not degrade, or would not degrade as much).Stated differently, this compensation decreases the probability that alist will change, given larger documents. For example, if for a documentof “n” words, it was desired to have a 50% chance that a change of n/10words would lead to the document not sharing any common fingerprints,the expression:

0.5=1−(1−(1−p)^((n/10)))^(x)

could be solved for p to get a function for computing p for a documentsize of n that gives the desired results.

§ 4.3.2.3 Exemplary Fingerprint Generation Method

FIG. 6 is a high-level flow diagram of an exemplary method 360′ that maybe used to effect the fingerprint generation operation 360. As indicatedby block 610, for a given document, the document identifier and itsassociated lists (as populated with extracted parts) are accepted. Then,as indicated by the loop 620-640 through all of the accepted lists, afingerprint to the current list is generated (See block 630.). After allof the lists are processed, the document identifier is associated withthe generated fingerprints, as indicated in block 650, before the method360′ is left via RETURN node 660.

Referring back to block 630, fingerprinting may simply be a stronglycollision-free hashing function, or a hashing function with a lowprobability of collision. The present invention may use any one of anumber of known fingerprinting methods. See, e.g., the article M. 0.Rabin, “Fingerprinting by Random Polynomials”, Report TR-15-81, Centerfor Research in Computing Technology, Harvard University (1981)(hereafter referred to as “the Rabin article”). See also, the article A.Z. Broder, “Some Applications of Rabin's Fingerprinting Method,” foundin the text R. Capocelli et al., editors, Sequences II: Methods inCommunications, Security, and Computer Science, pp. 143-152,Springer-Verlag (1993) (hereafter referred to as “the Broder Fingerprintarticle”).

The fingerprinting function used for processing lists may be made to beorder sensitive (i.e., sensitive to the order of parts (e.g., words) ina list), or not.

§ 4.3.2.4 Exemplary Near-Duplicate Detection Method

FIG. 7 is a high-level flow diagram of an exemplary method 376′ that maybe used to effect the near-duplicate detection operation 376. Thismethod 376′ can be provided with any two documents to be analyzed forpurposes of determining whether or not they are near-duplicates. Asindicated by block 710, document fingerprints for a first document andthose for a second document are accepted. Further, as indicated by block720, a NEAR_DUP flag may be initialized to “False”. Then, as indicatedby the loop 740-760 through each fingerprint of the second document,nested within the loop 730-770 through each fingerprint of the firstdocument, the current fingerprints are compared to determine whether ornot they match (See decision branch point 750.). If the currentfingerprints of the first and second documents match, a near duplicateindicator is set (e.g., the NEAR_DUP flag is set to “True”) as indicatedby block 752. In one embodiment, one of the two documents may then bedeleted as indicated by phantom block 754, before the method 376′ isleft via RETURN node 780. Referring back to decision branch point 750,if, on the other hand, the current fingerprints of the first and seconddocuments do not match, a next fingerprint of the second document isused, as indicated by loop 740-760. Once all of the fingerprints of thesecond documents have been tried with a given fingerprint of the firstdocument, a next fingerprint of the first document is used, as indicatedby loop 730-770. Once all combinations of the fingerprints of the firstand second documents have been tried, the method 376′ is left via theRETURN node 780. Note that if no matches are found, the documents areindicated as not being near-duplicates (e.g., NEAR_DUP flag remains setto “False”). A near-duplicate indicator may be provided for eachpossible pair of documents.

In a collection of documents, a document-fingerprint pair for each ofthe at least two fingerprints may be generated for each of thedocuments. Such fingerprint-document pairs may then be sorted based onvalues of the fingerprints. In this way, only documents with matchingfingerprints need be analyzed.

§ 4.3.2.5 Other Exemplary Methods

FIG. 8 is a high-level flow diagram of an exemplary method 382′ that maybe used to effect the document cluster determination operation 382′. Asindicated by the loop 810-870 through each of the (unprocessed)documents, a number of actions are taken. As indicated by the loop820-840, nested within loop 810-870, through each previously processeddocument, it is determined whether or not a current document is anear-duplicate of a current previously processed document (See decisionbranch point 830.). If so, it is determined whether or not the currentdocument is already a member of a cluster (with other documents), asdepicted in conditional branch point 860. If not, the current documentis associated with the cluster to which the current previously processeddocument belongs (e.g., is associated with the cluster ID of the currentpreviously processed document) as indicated by block 862, and the method382′ continues to block 840. If, on the other hand, the current documentis already a member of a cluster, then the two clusters are merged(e.g., by associating each member of the cluster to which the currentdocument belongs with the cluster identifier of the current previouslyprocessed document), as indicated by block 864, and the method 382′continues to block 840.

Referring back to decision branch point 830, if, on the other hand, thecurrent document is not a near-duplicate of the current previouslyprocessed document, as indicated by loop 820-840, a next previouslyprocessed document is tried. If, however, there are no more previouslyprocessed documents, the current document is associated with a newcluster (e.g., a new cluster identifier is created and the currentdocument is associated with it), as indicated by block 850. Then,another (unprocessed) document is processed as indicated by loop810-870. If there are no more unprocessed documents, the method 382′ isleft via RETURN node 880.

FIG. 9 is a high-level block diagram of an exemplary method 384′ thatmay be used to effect the query-responsive near-duplicate detectionoperation 384. As indicated by block 910, a group of candidate searchresults is accepted (e.g., from a searching operation 230). As indicatedby the loop 920-960 through (unprocessed) candidate search results, anumber of actions are taken. As indicated by the loop 930-950 througheach of the previously processed candidate search results, which isnested within the loop 920-960, it is determined whether or not thedocument associated with the current candidate search result and thedocument associated with the current previously processed candidatesearch result are near-duplicates (e.g., belong to the same cluster)(See decision branch point 940.).

If the document associated with the current candidate search result andthe document associated with the current previously processed candidatesearch result are near-duplicates (e.g., belong to the same cluster),then the current candidate search result is removed from the group ofcandidate search results as indicated by block 942. As indicated byoptional block 944, a next highest ranking candidate search result maybe added to the group. Thus, for example, if a search operation returnssearch results in groups of ten, and if a document associated with afifth candidate result is a near-duplicate of (e.g., belongs to the samecluster as) a document associated with a second candidate result, thefifth candidate result may be removed (leaving nine candidate results),and a next highest ranking (eleventh) candidate search result may beadded to the group (resulting in ten candidate search results). Another(unprocessed) candidate search result is checked as indicated by loop920-960. If there are no more (unprocessed) candidate search resultsleft, then the method 384′ is left via RETURN node 970.

Referring back to conditional branch point 940, if, on the other hand,the current candidate search result and the document associated with thepreviously processed candidate search result are not near-duplicates(e.g., do not belong to the same cluster), then a next previouslyprocessed candidate search result is tried as indicated by loop 930-950.If there are no more previously processed candidate search results left,another (unprocessed) candidate search result is checked as indicated byloop 920-960. If there are no more (unprocessed) candidate searchresults left, then the method 384′ is left via RETURN node 970.

Having described various exemplary methods that may be used to effectvarious operations, exemplary apparatus for effecting at least some ofsuch operations are described in § 4.3.3 below.

§ 4.3.3 Exemplary Apparatus

FIG. 10 is high-level block diagram of a machine 1000 that may effectone or more of the operations discussed above. The machine 1000basically includes a processor(s) 1010, an input/output interfaceunit(s) 1030, a storage device(s) 1020, and a system bus or network 1040for facilitating the communication of information among the coupledelements. An input device(s) 1032 and an output device(s) 1034 may becoupled with the input/output interface(s) 1030.

The processor(s) 1010 may execute machine-executable instructions (e.g.,C or C++ running on the Solaris operating system available from SunMicrosystems Inc. of Palo Alto, Calif. or the Linux operating systemwidely available from a number of vendors such as Red Hat, Inc. ofDurham, N.C.) to effect one or more aspects of the present invention. Atleast a portion of the machine executable instructions may be stored(temporarily or more permanently) on the storage device(s) 1020 and/ormay be received from an external source via an input interface unit1030.

Some aspects of the present invention may be effected in the generalcontext of computer-executable instructions, such as program modules,being executed by a personal computer. However, the methods of thepresent invention may be effected by (and the data structures of thepresent invention may be stored on) other apparatus. Program modules mayinclude routines, programs, objects, components, data structures, etc.that perform an operation(s) or implement particular abstract datatypes. Moreover, those skilled in the art will appreciate that at leastsome aspects of the present invention may be practiced with otherconfigurations, including hand-held devices, multiprocessor systems,microprocessor-based or programmable consumer electronics, networkcomputers, minicomputers, set-top boxes, mainframe computers, and thelike. At least some aspects of the present invention may also bepracticed in distributed computing environments where tasks areperformed by remote processing devices linked through a communicationsnetwork. In a distributed computing environment, program modules may belocated in local and/or remote memory storage devices.

In one embodiment, the machine 1000 may be one or more conventionalpersonal computers. In this case, the processing unit(s) 1010 may be oneor more microprocessors, the bus 1040 may include a system bus thatcouples various system components including a system memory to theprocessing unit(s). The system bus 1040 may be any of several types ofbus structures including a memory bus or memory controller, a peripheralbus, and a local bus using any of a variety of bus architectures. Thestorage devices 1020 may include system memory, such as read only memory(ROM) and/or random access memory (RAM). A basic input/output system(BIOS), containing basic routines that help to transfer informationbetween elements within the personal computer, such as during start-up,may be stored in ROM. The storage device(s) 1020 may also include a harddisk drive for reading from and writing to a hard disk, a magnetic diskdrive for reading from or writing to a (e.g., removable) magnetic disk,and an optical disk drive for reading from or writing to a removable(magneto-) optical disk such as a compact disk or other (magneto-)optical media. The hard disk drive, magnetic disk drive, and (magneto-)optical disk drive may be coupled with the system bus 1040 by a harddisk drive interface, a magnetic disk drive interface, and an (magneto-)optical drive interface, respectively. The drives and their associatedstorage media may provide nonvolatile storage of machine-readableinstructions, data structures, program modules and other data for thepersonal computer. Although the exemplary environment described hereinemploys a hard disk, a removable magnetic disk and a removable opticaldisk, those skilled in the art will appreciate that other types ofstorage media (with appropriate interface devices), may be used insteadof, or in addition to, the storage devices introduced above.

A user may enter commands and information into the personal computerthrough input devices 1032, such as a keyboard and pointing device(e.g., a mouse) for example. Other input devices such as a microphone, ajoystick, a game pad, a satellite dish, a scanner, or the like, may also(or alternatively) be included. These and other input devices are oftenconnected to the processing unit(s) 1010 through a serial port interface1030 coupled to the system bus 1040. Input devices may be connected byother interfaces 1030, such as a parallel port, a game port or auniversal serial bus (USB). However, in the context of a search facility130, no input devices, other than those needed to accept queries, andpossibly those for system administration and maintenance, are needed.

The output device(s) 1034 may include a monitor or other type of displaydevice, which may also be connected to the system bus 1040 via aninterface 1030, such as a video adapter for example. In addition to (orinstead of) the monitor, the personal computer may include other(peripheral) output devices (not shown), such as speakers and printersfor example. Again, in the context of a search facility 130, no outputdevices, other than those needed to communicate query results, andpossibly those for system administration and maintenance, are needed.

The computer may operate in a networked environment which defineslogical and/or physical connections to one or more remote computers,such as a remote computer. The remote computer may be another personalcomputer, a server, a router, a network computer, a peer device or othercommon network node, and may include many or all of the elementsdescribed above relative to the personal computer. The logical and/orphysical connections may include a local area network (LAN) and a widearea network (WAN). An intranet and the Internet may be used instead of,or in addition to, such networks.

When used in a LAN, the personal computer may be connected to the LANthrough a network interface adapter (or “NIC”) 1030. When used in a WAN,such as the Internet, the personal computer may include a modem or othermeans for establishing communications over the wide area network. In anetworked environment, at least some of the program modules depictedrelative to the personal computer may be stored in the remote memorystorage device. The network connections shown are exemplary and othermeans of establishing a communications link between the computers may beused.

Referring once again to FIG. 1, the information access facility 110 maybe a personal computer, the browsing operation 112 may be an Internetbrowser such as Explorer from Microsoft Corporation or Netscape from SunMicrosystems, and the input/output interface operation(s) 118 mayinclude communications software and hardware. Other information accessfacilities 110 may be untethered devices such as mobile telephones,personal digital assistants, etc., or other information appliances suchas set-top boxes, network appliances, etc.

§ 4.4 Examples of Operations of Exemplary Embodiment

Examples of operations of an exemplary embodiment of the presentinvention is now described with reference to FIGS. 11 through 18.

FIG. 11 is an example illustrating an operation of an exemplaryextraction operation. A document 320 a is shown in canonical form. Anextraction operation 340′ uses a “word” unit type, a part size of oneword, and no (zero word) overlap. As shown, the extraction operation340′ returns document extracted parts 345 a including a documentidentifier 1110 and parts 1120.

In another example, suppose that the document is “ABCDE”, a “character”unit type is used, a part size is three characters, and an overlap isone character. In this second example, the extracted parts would be“ABC”, “BCD” and “CDE”.

FIGS. 12A and 12B, collectively, provide an example illustrating anoperation of an exemplary list population operation 350′. In thisexample, the number of lists is set to four, the document partsextracted in the example of FIG. 11 are processed, and each part goes toone and only one list. FIG. 12A illustrates the four lists 1220 aspopulated after four parts (e.g., words) have been processed. Note thata document identifier 1210 is associated with the lists. FIG. 12Billustrates the four lists 1220′ as populated after eight parts (e.g.,words) have been processed. Note that since the (e.g., hashing) processused to determine which list the part is to be sent is repeatable andnot sensitive to state, the second “the” part is sent to the same list1220 c′ as the first “the” part.

FIG. 13 illustrates a Web page 1300 of eight results 1310-1380 to asearch query 1390. In this example, the search results 1310-1380 includea title of a corresponding document, a hyper-text link to thecorresponding document, and snippets from the corresponding document.These search results 1310-1380 would be returned (and rendered to anend-user) if a query-responsive near-duplicate detection operation werenot used. FIG. 14 illustrates the document (e.g., Web page) 1310′associated with the first search result 1310, FIG. 15 illustrates thedocument (e.g., Web page) 1320′ associated with the second search result1320, FIG. 16 illustrates the document (e.g., Web page) 1330′ associatedwith the third search result 1330, FIG. 17 illustrates a document (e.g.,Web page) 1340′ associated with the fourth search result 1340, and FIG.18 illustrates a document (e.g., Web page) 1350′ associated with a fifthsearch result 1350. Clearly, these five documents are near-duplicates ofone another. Most users would not want to see the others after seeingone, since no additional useful information would be conveyed. Indeed,these documents (Web pages) differ only in the date the page wasretrieved (e.g., by a crawler operation 202) (“Monday, July 3”, or“Tuesday, July 4”) the category for the page (“Home: Personal Care:Aeron Chair”, or “Home: Back Care: Chairs: Aeron Chair”), and/or thetitle (“aeron chair.aeron, aerons, ergonomic chairs”, or “Herman MillerAERON Chair from AHC.herman, miller”). An exemplary query-responsivenear-duplicate detection method 384′ could be used to remove, the second1320, third 1330, fourth 1340, and fifth 1350 candidate search resultsand add other search results to the group of search results to bereturned (and rendered to the end-user).

CONCLUSIONS

As can be appreciated from the foregoing, improved near-duplicatedetection techniques are disclosed. These near-duplicate detectiontechniques are robust, and reduce processing and storage requirements.Such reduced processing and storage requirements is particularlyimportant when processing large document collections.

The near-duplicate detection techniques have a number of importantpractical applications. In the context of a search engine for example,these techniques can be used during a crawling operation to speed-up thecrawling and to save bandwidth by not crawling near-duplicate Web pagesor sites, as determined from documents uncovered in a previous crawl.Further, by reducing the number of Web pages or sites crawled, thesetechniques can be used to reduce storage requirements of a repository,and therefore, other downstream stored data structures. These techniquescan instead be used later, in response to a query, in which case a useris not annoyed with near-duplicate search results. These techniques mayalso he used to “fix” broken links. That is, if a document (e.g., a Webpage) doesn't exist (at a particular location or URL) anymore, a link toa near-duplicate page can be provided.

1. A computer-implemented method for determining whether two documentsare near-duplicates, the computer-implemented method comprising: a) foreach of the two documents, generating at least two differentfingerprints; and b) determining whether or not the two documents arenear-duplicate documents by 1) determining whether or not any one of theat least two fingerprints of a first of the two documents matches anyone of the at least two fingerprints of a second of the two documents,and 2) if it is determined that any one fingerprint of the at least twofingerprints of the first of the two documents does match any onefingerprint of the at least two fingerprints of the second of the twodocuments, then concluding that the two documents are near-duplicates;and c) using the determination of whether or not the two documents arenear-duplicates in at least one of (A) an act of serving search resultscorresponding to documents, (B) an act of crawling documents, (C) an actof indexing documents, and (D) an act of fixing a broken link to atleast one of the two documents.
 2. A machine-readable medium havingstored thereon a plurality of records, each of the records comprising:a) a first field for storing a document identifier; and b) a pluralityof lists, each of the plurality of lists containing elements of adocument identified by the document identifier stored in the firstfield, wherein each of the elements are contained in one of theplurality of lists in accordance with a result of hashing the elementusing a hash function.
 3. The machine-readable medium of claim 2 whereineach of the elements of a document is an element that has been extractedfrom the document.
 4. The machine-readable medium of claim 2 whereineach of the elements of a document is a predetermined number of words.5. The machine-readable medium of claim 2 wherein each of the elementsof a document is a predetermined number of sentences.
 6. Themachine-readable medium of claim 2 wherein each of the elements of adocument is a predetermined number of characters.
 7. Themachine-readable medium of claim 2 wherein each of the elements of adocument is a predetermined number of paragraphs.
 8. Themachine-readable medium of claim 2 wherein each of the elements of adocument is a predetermined number of sections.
 9. The machine-readablemedium of claim 2 wherein each of the elements of a document partiallyoverlaps another of the elements of the document.
 10. Themachine-readable medium of claim 2 wherein at least some of theplurality of lists include different numbers of elements.
 11. Themachine-readable medium of claim 2 wherein the at least one of theplurality of records includes at least one list including no elements.12. The machine-readable medium of claim 2 wherein at least somecontiguous elements in a document are not contiguous elements of a list.13. The machine-readable medium of claim 2 wherein for each of therecords, the number of lists is the same.
 14. Apparatus for determiningwhether two documents are near-duplicates, the apparatus comprising: a)at least one processor; and b) at least one storage device storing aprocessor executable program which, when executed by the at least oneprocessor, performs a method including 1) for each of the two documents,generating at least two different fingerprints; and 2) determiningwhether or not the two documents are near-duplicate documents by A)determining whether or not any one of the at least two fingerprints of afirst of the two documents matches any one of the at least twofingerprints of a second of the two documents, and B) if it isdetermined that any one fingerprint of the at least two fingerprints ofthe first of the two documents does match any one fingerprint of the atleast two fingerprints of the second of the two documents, thenconcluding that the two documents are near-duplicates; and C) using thedetermination of whether or not the two documents are near-duplicates inat least one of (i) an act of serving search results corresponding todocuments, (ii) an act of crawling documents, (iii) an act of indexingdocuments, and (iv) an act of fixing a broken link to at least one ofthe two documents.